Multispectral imaging scanners are commonly mounted on moving aircraft or satellites for the purpose of recording images of earth terrain as they pass over the earth's surface. The images are recorded as data which consist of a series of scan lines whose relative orientation with respect to other scan lines may change as the aircraft or spacecraft moves along its line of flight. The attitude of aircraft and spacecraft is subjected to perturbations induced by roll, pitch, yaw, altitude, and velocity changes which introduce non-systematic geometric distortions into the image of the terrain. In the case of aircraft, such perturbations result from wind buffeting, changes in air density and inadvertent course changes. Perturbations in a satellite environment are more subtle, but can also result from atmospheric buffeting (in low altitude orbit), orbital control maneuvers, and changes in the satellite's center of mass (i.e. fuel consumption, antenna and solar array orientation changes).
In order to correct the geometry of the "raw" image data, it is usually possible to identify pixels with ground control points on maps for every few scan lines in the image data by searching maps for the exact location of features which are distinct pixels (picture elements) in the image data, then mathematically warping and resampling the entire image to arrive at the best fit for all the ground control points. However, this approach is especially expensive in terms of the time and labor required to manually correlate the recorded image data with the position of ground control points, and the correction relies exclusively upon the accuracy and density of the ground control. Another solution to this problem involves horizon seekers, star pointers and inertial navigation systems on-board the spacecraft or aircraft. These attitude sensors detect and record attitude change data required for the partial geometric correction of image data. Such data is not optimal for at least three reasons: first, they use features or inertial references which are not an intrinsic part of the recorded terrain data; second, they can cost as much as the primary sensor; and third, their weight and power consumption maybe prohibitive for some applications.
Consequently, there is a need in the art for a lightweight, more efficient, and less expensive system for detecting and recording the attitude changes of a platform, such as an imaging sensor platform, which can be used either for aircraft or satellite terrain image sensor applications.
The problems of accurately determining changes in the attitude of an airborne or spaceborne platform discussed above are also applicable to other uses of such platforms, such as mounting an energy beam pointing device which may comprise, for example a mirror arrangement to reflect energy beams, or an antenna employed to receive or transmit energy in the radio frequency spectrum. In many applications, minute changes in the attitude of the beam pointing platform can have a major impact on the aiming accuracy of the beam pointer where extremely precise pointing accuracy is required. Attitude control systems for correcting and altering the attitude of beam pointer mechanisms of course require dynamic, real time control, thus dictating a system capable of quickly detecting minute changes in platform attitude and dynamically controlling servomotors quickly after the attitude change has been sensed in order to re-aim the energy beam.
The above problems of determining changes in platform attitude are exacerbated by atmospheric or geological characteristics that interfere and sometimes prevent a complete "snap shot" image from being formed of the entire area of the earth beneath the aircraft or spacecraft. For example, the area of the ground within the field of view beneath the spacecraft or aircraft may not be completely visible due to cloud cover or the presence of a body of water, which make it impossible to measure changes in platform and attitude and altitude, even through there may be some area on the ground within the field of view of the imaging sensor which exists either between the clouds or on either side of the body of water for which imaging data can be generated. However, in the past, under these circumstances, the data so acquired by the primary imaging sensor corresponding to images within the sensor's field of view that are between clouds or on the other side of a body of water is discarded because the overall data set for the entire field of view does not possess sufficient quality for mapping purposes. Accordingly, it would be desirable to employ a method of determining and altering the attitude of the platform which is not dependent upon requiring an entire set of "good" mapping data from the primary sensor's full field of view, but rather is effective through the use of only a partial data set, the data corresponding to images of that portion of the ground within the field of view that can indeed by "seen" by the sensor which is not otherwise obscured.